The measurement of the total organic carbon (TOC) concentration, total inorganic carbon (TIC) concentration, and total carbon (TC) concentration in water has become a standard method for ascertaining the level of contamination by organic and inorganic carbon compounds in potable waters, industrial process waters, and municipal and industrial waste waters. In addition to widespread terrestrial applications, the measurement of TOC is one of the primary means for determining the purity of potable and process waters for manned space based systems including the space shuttle, the proposed space station and for future manned explorations of the moon and other planets.
A variety of prior art approaches for measuring the total organic carbon content of water have been proposed, for example, in U.S. Pat. No. 3,958,941 (Regan); U.S. Pat. No. 3,224,837 (Moyat); U.S. Pat. No. 4,293,522 (Winkler); U.S. Pat. No. 4,277,438 (Ejzak); U.S. Pat. Nos. 4,626,413 and 4,666,860 (Blades et al.); U.S. Pat. No. 4,619,902 (Bernard); U.S. Pat. No. 5,275,957 (Blades et al.); and U.S. Pat. Nos. 5,132,094 and 5,443,991 (Godec et al.), each of which is incorporated herein by reference.
Representative of the devices described in these references are the methods described in U.S. Pat. No. 3,958,941 (Regan). In Regan an aqueous sample is introduced into a circulating water stream that flows through a reaction chamber where the sample is mixed with air and exposed to ultraviolet (U.V.) radiation to promote the oxidation of organic compounds to form carbon dioxide. The carbon dioxide formed in the reaction chamber is then removed from solution by an air stripping system and introduced into a second chamber containing water that has been purified to remove ionic compounds. The conductivity of the water in the second chamber is measured, and any increase in conductivity is related to the concentration of carbon dioxide formed in the first reactor. The conduction measurement can be used, therefore, to determine the concentration of organic compounds in the original sample.
But, the Regan device is slow, cannot be used for the continuous monitoring of TOC concentration in flowing aqueous streams, cannot be scaled down without increasing interference from commonly-occurring contaminants, such as NO.sub.2, SO.sub.2 and H.sub.2 S, to unacceptable levels, and is therefore generally unsatisfactory. In addition, Regan does not disclose that an aqueous solution of acid must be added to the sample stream to reduce the pH to a value of less than about 4 to insure a reasonable removal rate of carbon dioxide using the air stripping system described. The oxidation method disclosed by Regan is unsatisfactory for the measurement of refractory compounds, particularly urea. In Regan, an aqueous sample of 20 to 100 mL containing 0.5 mg/L organic carbon is required to generate sufficient carbon dioxide for accurate detection, thus limiting the utility of the device for the measurement of sub-part per million levels of TOC in smaller sample sizes. Finally, in practice, the Regan system requires frequent recalibration-typically once per day--due to variations in background conductivity. Also, the concentration of total organic carbon in the calibration standard must be approximately equal to the concentration of organic carbon in the sample. Because of this, recalibration is required when analyzing aqueous samples containing higher or lower levels of organic carbon when compared with the calibration standard.
Another method and apparatus for the measurement of organic content of aqueous samples is that described in U.S. Pat. No. 4,277,438 (Ejzak). Ejzak describes a multistage reactor design which provides for the addition of oxygen and a chemical oxidizing agent, preferably sodium persulfate, to the aqueous sample stream prior to oxidation of the stream using ultraviolet radiation in a series of reactors. Ejzak also describes the use of an inorganic carbon stripping process--before oxidation of the organic carbon--that includes the addition of phosphoric acid to the sample stream. After oxidation, the sample stream is passed into a gas-liquid separator where the added oxygen acts as a carrier gas to strip carbon dioxide and other gases from the aqueous solution. In the preferred embodiment, the gas stream is then passed through an acid mist eliminator, a coalescer and salt collector, and through a particle filter prior to passage into an infrared (IR) detector for the measurement of the concentration or carbon dioxide in the gas stream.
The methods and apparatus disclosed by Ejzak provide certain improvements over the Regan patent; however, the Ejzak device requires extensive manual operation and is generally unsatisfactory for other reasons as well. Thus, the Ejzak device requires three external chemical reagents: oxygen gas, aqueous phosphoric acid and an aqueous solution of sodium persulfate. Both the phosphoric acid and persulfate solutions must be prepared at frequent intervals by the operator due to the relatively high rate of consumption. The Ejzak device requires dilution of the sample if the solution contains high concentrations of salts in order to insure complete oxidation of the sample and to eliminate fouling of the particle filter located prior to the IR carbon dioxide detector. As in the Regan patent, relatively large sample sizes are required--typically 20 mL of sample for accurate measurement at 0.5 mg/L total organic carbon--and the carbon dioxide formed in the oxidation chamber is removed using a gravity dependent technique that cannot be easily used in space-based operations.
Still another method and apparatus for the measurement of total organic carbon in water is disclosed in U.S. Pat. No. 4,293,522 (Winkler). In Winkler, an oxidizing agent, specifically molecular oxygen, is generated in situ by the electrolysis of water. Organic compounds are subsequently oxidized to form carbon dioxide by a combination of exposure to U.V. radiation and the in situ-generated oxygen. Winkler does not teach or suggest, however, that the aqueous sample stream be acidified to assist in the removal of carbon dioxide from solution. On the contrary, Winkler teaches away from the use of acid. Therefore, the Winkler method and apparatus cannot be used for high accuracy measurement of very low levels of organic compounds in basic aqueous samples. Also, the oxidation chamber of Winkler uses a solid electrolyte to separate the two electrodes employed for the electrolysis of water. The solid electrolyte described by Winkler is composed of an organic polymer which, under exposure to oxygen, ozone and U.V. radiation, will undergo oxidation to form carbon dioxide, therefore resulting in unacceptable and misleading background levels of carbon and/or organic compounds in the sample stream. These background levels of carbon and/or organic compounds, though typically small, become proportionally very large and increasingly significant sources of error at very low organic compound concentrations in the sample.
Winkler also describes a conductometric carbon dioxide detection system wherein the sample stream exiting the oxidizing chamber must be held in an equilibriating relationship to a stream of deionized water. The two flowing streams are separated by a CO.sub.2 permeable membrane that allows the concentration of carbon dioxide to equilibrate between the streams. The concentration of carbon dioxide is then determined by measuring the conductance of the deionized water stream which has absorbed CO.sub.2 which has diffused through the membrane. However, the use of two flowing streams introduces operating parameters into the detection process resulting in the need for frequent calibration adjustments.
Another example of the prior art in this field is U.S. Pat. No. 4,619,902 (Bernard), which teaches the oxidation of organic compounds to form carbon dioxide using persulfate oxidation at elevated temperatures--typically 20.degree. to 100.degree. C.--in the presence of a platinum metal catalyst. Bernard recognizes that the materials used in the construction of instrumentation for the determination of total organic carbon in water can contribute organic compounds to the sample during the measurement process, and teaches that inert materials, such as polytetrafluoroethylene (PTFE), must be used to minimize this background interference. As with the previously mentioned patent, a gas stripping technique is employed to collect the formed carbon dioxide, and measurement is made using IR spectrometry. Bernard also recognizes that aqueous solutions of sodium persulfate are not stable due to auto-degradation of the reagent, thus requiring fresh supplies.
Another system for the measurement of organic compounds in deionized water is described in U.S. Pat. No. 4,626,413 (Blades and Godec). The apparatus described by Blades and Godec is based on direct U.V. oxidation of organic compounds to form carbon dioxide, which is then measured by using conductometric detection. In the Blades and Godec patent, the oxidation of some organic compounds results in the formation of strong acids, such as HCl, H.sub.2 SO.sub.4 and HNO3, which then interfere with the conductometric measurements. The Blades and Godec patent is also limited to the measurement of total organic compounds in deionized water and cannot be used for samples containing ionic compounds other than bicarbonate ion. Additionally, the levels of TOC detection are limited by the availability of dissolved oxygen in the sample and the small amounts of hydroxyl radicals generated from the photolysis of water from 185 nm radiation.
U.S. Pat. No. 4,626,413 (Blades and Godec) is also the parent of a series of subsequent patents, each based at least in part, on the parent case, but also adding additional disclosure and refinements of various types. Included in this series of subsequent related patents are U.S. Pat. Nos. 4,666,860; 5,047,212; and 5,275,957. The latter patent suggests, for example, that electrophoresis can be used to speed the reaction, but it fails to teach the types of electrolytic oxidation cells which are the subject of this invention.
In U.S. Pat. No. 4,209,299 (Carlson), it is disclosed that the concentration of volatile materials in a liquid can be quantitatively determined by transferring the desired material through a gas permeable membrane into a liquid of known conductivity, such as deionized water. The Carlson device is demonstrated for the measurement of a number of volatile organic and inorganic compounds, but Carlson does not suggest the combination of this process in conjunction with a carbon dioxide producing reactor.
The use of aqueous solutions of persulfate salts for the oxidation of organic compounds is widely known. For example, Smit and Hoogland (16 Electrochima Acta, 1-18 (1971)) demonstrate that persulfate ions and other oxidizing agents can be electrochemically generated. Also, U.S. Pat. No. 4,504,373 (Mani et. al.), describes a method for the electrochemical generation of acid and base from aqueous salt solutions.
In electrochemical reactions in aqueous solutions, a common reduction product is hydrogen gas. Because of its flammability, the hydrogen presents a potential hazard in devices using electrochemical techniques. The interaction of hydrogen gas in aqueous solutions and palladium metal is well known (e.g., F. A. Lewis, "The Palladium Hydrogen System," Academic Press, 1967, London, incorporated herein by this reference); and, the use of palladium offers a potential solution to the generation of hydrogen in electrochemical reactions by selective removal and disposal of the hydrogen.
The foregoing prior art processes and apparatus, however, have been unable to meet the increasingly demanding industry standards for ultrapure water, for example in pharmaceuticals, semiconductors and other such applications. Accurate measurement of carbon (as TOC or total organic carbon) in the 50-1000 parts per billion range, for example, is required to support new developments in semiconductor manufacturing. But, the prior art technology cannot accurately measure TOC in ultrapure water, which typically has a very low level of dissolved oxygen that is insufficient to oxidize all of the organic compounds in a sample to carbon dioxide. The result is an inaccurate and misleadingly low reading which suggests the water sample is purer than in fact is the case. This problem is exacerbated when the water sample contains an excess of residual hydrogen gas (e.g., 40 to 100 ppb H.sub.2), as might be found in one of the catalytic removal unit processes in semiconductor manufacture. In these cases, carbon measurements made using prior art technology may reflect as little as 18% to 22% of the correct value.
Adding sufficient oxygen to the water sample either before or during the analysis without also adding contaminants and creating other process problems, however, is not easy to accomplish. Thus, it is extremely difficult to diffuse enough additional oxygen obtained from air, or to just diffuse in ambient air, into the water sample without also introducing organic materials and/or atmospheric carbon dioxide, either of which would lead in inaccurate carbon measurements. In these cases, the carbon readings would suggest that the water sample was less pure than in fact is the case. Similarly, the addition of non-gaseous chemical oxidizing agents to a water sample raises possible problems of organic contaminants or introduces other process difficulties. Electrolysis of the ultrapure water in the sample itself could be a solution, but only as long as it can be done without any addition of extra organic compounds or carbon dioxide to the sample.